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March 5, 2007 -- Young girls are getting heavier, and they also appear to be entering puberty earlier than even 20 years ago. Now a new study provides some of the best evidence yet that the two trends are related.

Researchers found that girls who were overweight or at risk for overweight as early as age 3 were more likely to begin puberty earlier than normal-weight girls.

Also, a large increase in body fat through first grade, as measured by body mass index (BMI), was associated with earlier puberty.

"We have known that girls with higher body masses tend to develop earlier, but we haven't really understood if it is the weight gain that leads to early breast development or if it is the puberty that leads to weight gain," lead researcher Joyce Lee, MD, MPH, of the University of Michigan C.S. Mott Children's Hospital, tells WebMD.

The study's link between body fat and puberty lends support to the idea that the obesity epidemic among children in the U.S. may be driving a trend toward earlier puberty in girls, she adds.

Are Girls Really Maturing Earlier?

There is still some controversy about whether such a trend exists.

In 2005, researchers from Boston's Tufts University reported that the average age at which a girl gets her first period declined by 2.3 months in the U.S. between the late 1980s and 2002.

Other studies have suggested similar trends for breast and pubic hair development -- the two other measures of puberty in girls.

However, doubters attribute the perceived trend toward earlier development to more sophisticated tools for measuring the onset of puberty.

Those who believe the phenomenon is real point out that it has coincided with an increase in overweight and obesity among young girls.

In an effort to better understand the link between body weight and physical maturation, Lee and colleagues followed 354 young girls from the age of 3 through sixth grade. The girls had participated in a larger child development study and were chosen to reflect a wide range of social, economic, and regional backgrounds.

By fourth grade, 68% of the girls who were overweight or at risk for overweight had breast development indicative of puberty, compared with 40% of normal-weight girls, Lee says.

Higher BMI scores at all ages were strongly associated with earlier puberty.

And earlier onset of puberty was also associated with a bigger increase in body weight between age 3 and the first grade.

The findings are published in the March issue of Pediatrics.

'A Definite Risk Factor'

"Because the girls in the study were followed from such a young age, this makes it difficult to argue that the obesity we see in early maturing girls is due to puberty," says Paul B. Kaplowitz, MD, PhD, a pediatrics professor at George Washington University School of Medicine.

The author of the 2004 book Early Puberty in Girls, Kaplowiz's own research has found obesity to be an important contributor to earlier physical maturation in girls.

"I think that we can add early puberty to the long list of negative health consequences of obesity," Kaplowiz says. "But I don't really think this will alarm people into taking action. It is far more serious that we are seeing an epidemic of type 2 diabetes among children."

Nevertheless, earlier-onset puberty has been linked in some studies to increases in behavioral and psychosocial problems, earlier initiation of sex and alcohol abuse, and an increased risk for obesity in adulthood.

"Being overweight at an early age doesn't guarantee that a girl will mature early," Kaplowitz says. "Many overweight girls don't go through puberty early, and many girls who mature early are not overweight.

"But if you look at the big picture," he says, "this argues that weight is a definite risk factor for girls starting puberty early."